The “Russian period” of the life of Major Osman Bey (1836–1901?), stepson of Grand Vizier Kıbrıslı Mehmed Pasha, which covers the years from 1873 to 1882, gave him an opportunity to publish a number of works devoted to the Ottoman army. For example, in 1877 appeared the Russian translation of his memoirs of the Crimean War, which contain a narrative of military and political events in the Caucasus from the autumn of 1854 to the summer of 1855, including the so-called “Circassian expedition” of Field Marshal Alyanak Mustafa Pasha, the Commander of the Batum Ottoman army, in which Osman Bey took part as his aide-de camp. The present article deals with other war memoirs of his, namely the account of the final Russian siege of Kars during October-November of 1877, which can be found in his book “Les russes en 1877–1878 (Guerre d’Orient)” (Berlin, 1889). Since the book was published after his resignation from the Russian service it has been scarcely used by Russian historians. In the present article, special attention is paid to his relations with his old friend, Turkish Colonel Huseyin Bey, the then commander of the fortress artillery in Kars, and their contacts during the siege. The author traces Osman Bey’s account of the events in comparison with other historical sources. Among them the reports of a Russian war correspondent, the well-known military observer and historian Mikhail A. Terentiev and the memoirs of Prince Nikolai Tumanov are of particular importance. The work of Gazi Ahmed Muhtar Pasha “Sergüzeşt-i hayatım” (“The Adventures of my Life”), the then commander of the Anatolian Ottoman army, is also used. Besides, the author analyzes some reports of Western war correspondents such as Charles Williams and Charles Norman which are of importance for the study. The various explanations of the Turkish defeat, generated in both belligerent camps immediately after the fall of the fortress which had been previously regarded as impregnable, are treated. Among these opinions such explanations as treason, bribery, cowardice of the garrison and courage of the Russian army as well as the surprise night attack can be listed. Taking into consideration the fact that contradictory versions of the fall of Kars still co-exist in Russian historiography — a feature partly inherited from the pre-revolutionary past, the author considers the memoirs of Osman Bey as a valuable historical source which is worthy of further examination. Finally, the author argues that stressing the importance of moral factor in the warfare, Osman Bey shared to some extent the views of Leo Tolstoy which had been expressed in his famous novel “War and Peace”.